This “leaking toilet” question has been addressed many times on this Board. Someone always seems to raise the question of the following mechanisms: 1) cracked toilet bowl; 2) faulty venting, with other draining fixtures causing vacuum siphonage of the bowl; 3) wind over the roof vents causing a similar vacuum siphonage; and, 4) simple evaporation from the bowl.
Mechanism 4 is highly improbable. I have never seen it. Certainly standing water can evaporate, but even in a bone dry desert environment it would take many days (probably more than a month) for bowl water to evaporate.
Mechanisms 2 & 3 are possible, but except in the most poorly vented systems bowl drainage would be improbable and would likely be accompanied by an obvious suction of the bowl water into the drain. Also, such vacuum siphonage would be variable and clearly associated with winds over the roof or the use of other plumbing fixtures.
A cracked bowl should manifest as a leak, usually apparent on the floor around the bowl or from underneath (if you have access). It is possible for a cracked bowl to leak directly into the waste line without shedding water onto the floor or outside of the closet bend connection, but this sort of crack, though theoretically possible, would be extremely unlikely. It would have to involve a crack in the distal portion of the toilet trapway. I have been plumbing for more than 20 years and I have never seen such a crack…
In my opinion, the majority of these cases of “draining toilets” without manifest evidence of water seepage on the floor are due to partial obstructions of the trapway with material that is able to “wick” water out of the bowl and into the closet bend.
Thus, if you can rule out a cracked and leaky bowl, the next thing that should be done is to snake the bowl in place, or remove it and clear any obstructions from the trap. If you really want to exclude a cracked bowl, remove it and set it down somewhere, and fill it with water, preferably tinged with a dye. If it is really cracked, the water will drain out and leave a trace that you can follow. I can only hope that this post will answer these sorts of questions once and for all.
Thus Spake Zaraplumber…